parts of a computer

The classic crude oversimplication of a computer is that it contains three elements: processor unit, memory, and I/O (input/output). The borders between those three terms are highly ambigious, non-contiguous, and erratically shifting.
A slightly less crude oversimplification divides a computer into five elements: arithmetic and logic subsystem, control subsystem, main storage, input subsystem, and output subsystem.
processor
arithmetic and logic
control
main storage
external storage
input/output overview
input
output